\documentclass[12pt,a4paper]{report}
\begin{document}
\begin{equation}
	\left(
		\left[
			\left\{
				\sqrt{
					a+b
				}
			\right\}
			\right]
	\right)
\end{equation}

\ifnum\x<1
	\left[
		\left\{
			\sqrt{
				a+b
			}
		\right\}
		\right]
\fi

\left[
	\ifnum\x<1
		\left\{
			\sqrt{
				a+b
			}
		\right\}
	\fi
	\right]

\left[
	\begin{one}
		\left\{
			\sqrt{
				a+b
			}
		\right\}
	\end{one}
	\right]
\end{document}
